1. Field of the Invention
This invention relates to a recording device, and more particularly, to a recording device which uses a stepping motor as a driving source for moving a carriage mounting a recording head.
2. Description of the Prior Art
In a conventional serial-type recording device, a hybrid-type or PM (permanent magnet)-type stepping motor or brushless motor has been normally used as the motor for driving a carriage which carries a recording head for the purpose of recording scanning.
In a brushless motor, for example, Hall elements have been used for detecting positions of magnetic poles of a rotor to perform electric control, and an optical or magnetic encoder has been used for detecting the speed of the rotor.
However, such a conventional brushless motor has the following disadvanges: (1) It is required that magnetic poles of a stator are correctly positioned with respect to the Hall elements; and
(2) Since the positions of the Hall elements and the stator are uniquely determined when the current switching is effected by the Hall elements, a method for supplying current to the motor is limited to only one direction. For example, in the case of a so-called 180.degree. electric control, the positions of the Hall elements relative to the magnetic poles of the stator differ by 45.degree. electrically from those in the case of a so-called 90.degree. electric control. If two kinds of electric controls are effected by a single motor, the number of the Hall elements will be doubled and all of the Hall elements must be arranged in positions suitable for performing the respective electric controls.
Incidentally, Japanese Patent Laid-Open Nos. 62-193548 (1987) and 62-193549 (1987) each disclose a stepping motor wherein electric control is effected by utilizing an encoder output. However, these documents merely disclose the structure of the stepping motor itself including the encoder in a predetermined position, but do not disclose or teach the control circuit or method for driving the stepping motor.
U.S. Pat. No. 4,963,808 discloses a control apparatus for a stepping motor including an encoder having portions to be detected, the number of which is an integer multiple of the number of magnetic poles of the rotor, which is fixedly mounted on a shaft of the rotor. When the rotor is rotated, the number of the portions to be detected on the encoder passing by a predetermined position situated at the stator side are counted so that when the counted number coincides with a predetermined value the current supply to the coils of the stator is switched. Conventionally, the drive control for the stepping motor has been performed by merely performing an open-loop control of the number of driving pulses of the stepping motor and the frequency of the pulses.
However, if the stepping motor is used as the carriage driving motor and the stepping motor is driven by the open-loop control, during the movement of the carriage, discordant noise is generated due to the vibration of the rotor of the stepping motor, particularly in the case of the hybrid stepping motor. Further, upon starting, stopping and reversing the carriage, and accordingly, upon start, stop and reverse of the stepping motor, since the stepping motor is started or stopped with vibration, large noise is also generated. These noises must be avoided, particularly in an ink jet printer such as a bubble jet printer which generates no substantial noise.
U.S. Pat. No. 4,928,050 discloses a recording apparatus using the stepping motor as a driving source for scanning a recording head in a recording operation. The apparatus includes detection means for detecting the rotation angle of the rotor of the stepping motor, and control means for performing a closed-loop control of the drive of the stepping motor in accordance with the detection result by the detection means.
However, in order to perform a closed-loop control of the stepping motor, it is necessary to provide an encoder for detecting the rotation angle of the rotor and it is also necessary to register the positions of the magnetic poles of the rotor with the positions of the magnetic poles (slits in the magnetic or optical system) of the encoder during assembly of the stepping motor. The reason why such registration of positions between the magnetic poles of the rotor and those of the encoder is required is that the the phase switching of the stepping motor must be synchronous with the output pulses of the encoder. If such positional registration is not obtained with high accuracy, the motor will not rotate or will rotate at different rotational speeds in opposite directions.
On the other hand, if the number of pulses generated during one revolution of the encoder is increased to improve the resolving power for each pulse, such positional registration will not be required. For example, in a PM stepping motor in which one revolution is achieved by 48 steps, the number of the magnetic poles of the rotor is 24 (twenty-four). In this case, if the number of the output pulses of the encoder is 288 for each revolution, an output having 12 (twelve) pulses can be obtained for each magnetic pole of the rotor. If the encoder is fixedly mounted on the shaft of the rotor at random, since the deviation between the center of the magnetic poles of the rotor and the center of the magnetic poles of the encoder corresponds to a half of a distance of two adjacent pulses at the most, such deviation will be included in the range of .+-.4.2%. In this case, the deviation in the switching timing of the exciting current will be negligible.
However, in this case, it must be determined which magnetic pole of the encoder corresponds to the particular magnetic pole of the rotor. To this end, first of all, the current is supplied to the coils of the motor for at least a predetermined time period. Then, when the rotor of the motor is slightly rotated by the energization of the coils due to such current supply and then is stopped, the magnetic pole in the encoder which is registered with the magnetic pole of the rotor is selected. The other magnetic poles in the encoder may be selected at intervals of twelve pulses on the basis of the first selected magnetic pole.
The initialization of the encoder as mentioned above must be effected prior to the action of the stepping motor. That is to say, when such stepping motor is used as the carriage driving motor for a serial printer, it is necessary to initialize the encoder before the printer is powered on.
In order to perform such initialization, it has been previously proposed, in U.S. Ser. No. 413,473 filed on Sep. 27, 1989, (now U.S. Pat. No. 5,029,264 issued Jul. 2, 1991), in a recording apparatus to use a stepping motor as a driving source for scanning a recording head in a recording operation, a control device for the stepping motor which comprises detection means for detecting the rotation angle of the rotor of the stepping motor, and control means for performing a closed-loop control of the drive of the stepping motor in accordance with the detection result of the detection means and for driving the stepping motor and holding the rotor by controlling the current according to pulse-width modulation at the initialization processing wherein the drive of the stepping motor by the closed-loop control is started.
In such a device, since it is uncertain where the carriage is situated when power is supplied, the carriage is first moved within a range wherein the initialization can be securely executed.
At this time period, the stepping motor cannot perform closed-loop driving, but can move the carriage with step driving (open-loop driving). Subsequently, the above-described initialization is performed to make possible closed-loop driving. Since the positional relationship for switching the exciting current is thereafter maintained, the rotation of the motor can be continued by counting encoder signals.
However, in a serial-type recording device, since the operation of reversing the carriage is frequently performed in accordance with the printing operation, an abrupt change occurs in an encoder signal. A phenomenon thereby occurs wherein the position for switching the current deviates from a position set at the above-described initialization. As a result, according to a direction of rotation, an excessive current flows, the rotation speed is reduced, and rotation having vibration occurs, deteriorating the quality of printing. In the worst case, the motor stops.
It has also been proposed, in U.S. Ser. No. 551,796 filed on Jul. 12, 1990, (now U.S. Pat. No. 5,097,189 issued Mar. 17, 1992), to provide a recording device for driving a carriage by a stepping motor subjected to closed-loop control, which shifts the phase for switching the exciting current by detecting a change in the speed of the motor, to change the output torque characteristics of the motor, since, when the load torque applied to the carriage motor is increased due to a change in environment or a change in the course of time, the intended rise time, driving speed, amount of speed change and the like for starting the carriage cannot be satisfied.
However, even in such a recording device, no counter-measure is taken when miscounting of an encoder occurs at the reversal operation of the carriage, or the like.